The present invention relates to improvements in apparatus for optically testing or scanning rod-shaped commodities, and more particularly to improvements in apparatus for testing rod-shaped materials of the tobacco processing industry. Typical examples of rod-shaped materials which can be tested in such apparatus are continuous cigarette rods or filter rods or files or rows of discrete plain or filter cigarettes, filter rod sections or the like.
It is already known to equip an apparatus for optical testing of cigarette rods or the like with an annular support which surrounds a portion of the course or path for the moving rod and carries an annulus of light sources serving to direct infrared light upon successive increments of the external surface of the rod. The reflected light is directed upon an annulus of photosensitive detectors which transmit defect signals when the intensity of reflected light is changed as a result of the presence of a defect (such as a hole in the wrapper or a spot of adhesive). The signals are transmitted to an evaluating circuit which records the number of defects and/or generates signals for segregation of corresponding cigarettes from satisfactory cigarettes. The just outlined conventional apparatus can be used to detect defects in the form of holes in or smudges on the tubular wrapper of a cigarette rod or filter rod, the absence of improper application of imprints denoting the trademark, the trade name or other information pertaining to the manufacturer and/or to the product and spots of adhesive outside of the customary seam between the overlapping marginal portions of the wrapper. Spots of adhesive normally reflect more light than cigarette paper so that they effect an intensification of signals which are generated by the respective detector or detectors. On the other hand, holes in the wrapper consisting of cigarette paper or the like act not unlike dark spots and reduce the intensity of reflected light. Imprints influence the reflection of light to an extent which depends upon their color tone. Thus, the evaluating circuit which receives signals from the detectors can classify the incoming signals according to their intensity and hence according to the types of defects.
German Offenlegungsschrift No. 29 40 408 discloses a testing apparatus wherein light issuing from a source passes through a ring-shaped light-transmitting prism at an oblique angle to a plane which is normal to the axis of the tested rod-shaped material. Reflected light is intercepted by a set of light conducting elements in the form of glass fibers and is conducted to an optoelectrical transducer which generates signals whose intensity is proportional to the intensity of reflected light. Signals which are transmitted by the output of the transducer can be used to regulate the operation of the machine and/or to effect segregation of defective articles. The provision of a light source which directs light at an oblique angle with reference to the aforementioned plane is considered desirable and advantageous because it increases the contrasts which are attributable to irregularities in the region of defects, i.e., such orientation of incident light enhances the sensitivity of the testing apparatus. However, the just described testing apparatus also exhibits certain serious drawbacks which are attributable to the aforementioned orientation of incident light. Thus, light which impinges upon successive increments of the external surface of the moving rod-shaped material creates pronounced shadows whenever it encounters unevennesses in the external surface irrespective of whether or not such unevennesses are attributable to the presence of defects. The thus developing pronounced shadows are detected by the photosensitive transducer means and the latter transmits signals which lead to the segregation of the corresponding articles even though such articles are not defective at all. For example, slight undulations and/or minute creases in the wrapper of a cigarette rod are likely to induce the transducer means to generate defect signals even though the corresponding portion of the moving rod is not defective in a sense which would be irritating to the purchaser of cigarettes.
Another drawback of presently known testing apparatus is that they cannot detect all such defects which are detectable by the human eye and are likely to be irritating or unacceptable to the consumer. For example, a testing apparatus which operates with red or infrared light (such light is presently used in numerous testing apparatus for cigarettes or the like) cannot detect specks or smudges of oil or other fatty substances even though a purchaser will detect such defects and they are likely to deter him or her from making further purchases of the same brand.